BD6142AMUV Datasheet, PDF (8/28 Pages) Rohm – White Backlight LED Driver for Medium to Large LCD Panels (Switching Regulator Type)

BD6142AMUV
Technical Note
●Functional descriptions
1) PWM current mode DC/DC converter
While this IC is power ON, the lowest voltage of CH1, 2, 3, 4, 5, 6,7, 8 is detected, PWM duty is decided to be 0.8V
and output voltage is kept invariably. As for the inputs of the PWM comparator as the feature of the PWM current mode, one
is overlapped with error components from the error amplifier, and the other is overlapped with a current sense signal that
controls the inductor current into Slope waveform to prevent sub harmonic oscillation. This output controls internal Nch Tr
via the RS latch. In the period where internal Nch Tr gate is ON, energy is accumulated in the external inductor, and in the
period where internal Nch Tr gate is OFF, energy is transferred to the output capacitor via external SBD.
This IC has many safety functions, and their detection signals stop switching operation at once.
2) Pulse skip control
This IC regulates the output voltage using an improved pulse-skip. In “pulse-skip” mode the error amplifier disables
“switching” of the power stages when it detects low output voltage and high input voltage. The oscillator halts and the
controller skip switching cycles. The error amplifier reactivates the oscillator and starts switching of the power stages again
when this IC detects low input voltage.
At light loads a conventional “pulse-skip” regulation mode is used. The “pulse-skip” regulation minimizes the operating
current because this IC does not switch continuously and hence the losses of the switching are reduced. When the error
amplifier disables “switching”, the load is also isolated from the input. This improved “pulse-skip” control is also referred to
as active-cycle control.
PWM
VOUT
LX
duty 20% @1.25MHz(typ)
Pulse skip
LED current
20mA
Fig. 11 Pulse-skip
3) Soft start
This IC has soft start function.
The soft start function prevents large coil current.
Rush current at turning on is prevented by the soft start function.
After Enable, PWM is changed ‘L’ ‘H’, and UVLO is detected, soft start becomes effective for within typ 4.3ms and soft
start doesn't become effective even if Enable is changed ‘L’  ‘H’ after that.
Enable
PWM
VDC
Max 1ms ^{Typ 4.3ms}
UVLO
Soft start
OFF ON
OFF
ON
Enable
PWM
VDC
Soft start
Max 1ms Soft Start Time=T1+T2=4.3ms typ
UVLO
T1
T2
OFF ON OFF ON OFF
Fig. 12 Soft start
4) FAULT
When the error condition occurs, boost operating is stopped by the protection function, and the error condition is outputted
from FAULT. After power ON, when the protection function is operating under about 4.3ms(typ.) have passed.
Once enable change to ‘L’, FAULT status is reset
Object of protect function is as shown below.
Enable
PWM
- Over-voltage protection (OVP)
- Thermal shut down (OTP)
- Over current limit (OCP)
- Output short protect
- LED Short (Latch)
VDC
Typ4.3 ms
Mask
Typ100us
FAULT
‘X’
‘H’
Protection
un-detection
function(OVP, TSD, OCP)
‘L’
‘H’
‘L’
‘X’
‘H’
Latch
detect
un-detect
- LED Open (Latch)
Protection
un-detect
un-detect
function(LED open, LED short)
detect
Boost
operating
off
normal boost stop normal
off
normal
Fig. 13 FAULT operating description
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8/27
2011.07 - Rev.A